This invention relates to drive nut blind fasteners for use in fastening two panels together in overlapped outer and inner relation.
Blind fasteners are used in a variety of applications in which access to the blind side surface of panels being connected together is extremely limited or in some cases not possible. For example, in the construction of aerodynamic designs, including aircraft and the like, a substantially flush surface usually is desired on the accessible side of the panels, while access to the blind side may not be possible. Application and use of such fasteners in the aerospace industry impose some of the most stringent requirements which the fasteners must meet or exceed. In particular, the fasteners must securely fasten the panels but resist losing their gripping power under the stresses and vibrations imposed upon them by the harsh environment in which they are used.
One type of blind fastener that satisfies these requirements comprises an internally threaded fastener body for insertion into aligned holes of the two panels, and an externally threaded cylindrical stem passing in threaded engagement through the fastener body. The inserted end of the stem has an enlarged stem head, and the outer end of the stem has a wrench engaging portion. Upon turning motion of the stem relative to the fastener body, the stem is moved in an axial outward direction through the fastener body. This axial outward movement causes a deformable sleeve around the stem and abutting against the stem head to deform around the fastener body to a fully set condition against the inner panel. The stem further may be provided with a localized weakened region or break groove adapted to shear the stem at a predetermined torque. The break groove preferably is located axially along the stem such that the stem twists off in substantially flush relation to the outer portion of the fastener body, i.e., the fastener body head, after the fastener is fully set. In actual practice, the stem twists off within a range between approximately one-eighth of an inch above or below the outer surface of the fastener body head. The fastener body head normally is received in a countersunk, flush relationship to the outer panel, thus providing an aerodynamic surface after the fastener is set. A protruding fastener body head also may be used to engage the outer panel.
When using blind fasteners for securing panels together, as described above, a drive nut threaded on the outer portion of the stem may be provided for abutting contact with the fastener body head and gripping by an installation tool to hold it and the fastener body against rotation. When setting the fastener, slippage of the drive nut on the fastener body head occurs during rotation of the stem. With the relatively smooth surfaces of the drive nut and the fastener body head abutting, they tend to overtighten and jam together. As a result, relative turning slippage between them in a tightening direction is accommodated by elastic extension of the stem in the region gripped between the drive nut and the fastener body. This extension tends to develop a tensile load in the stem which, when added to the torsional load already acting on the stem from rotating the stem in a tightening direction, can cause the stem to twist off before the fastener is completely set and, therefore, undesirably subtract from the effective clamping force exerted by the fastener on the panels.
The blind fastener disclosed in application Ser. No. 828,781 to Pratt provides friction enhancing means on at least one of the abutting surfaces of the drive nut and fastener body head to reduce over tightening and jamming between the two elements and, therefore, the build-up of tensile load in the stem. This friction enhancing means is disclosed as a coating of high frictional resistance, or knurling of the surface. It does not, however, provide a positive mechanical engagement between the drive nut and the fastener body head.